You just can't walk from one surface directly onto another at a right angle, such as from the floor of a room onto a wall, directly. The game mechanics simply don't support that movement.

If the ammo indicator flashes red, it simply means that a zero grav jump to where your aiming is not possible from your current location. You are either aiming at a surface that doesn't support zero grav movement, or my guess is, you are standing too close to where you want to jump to.

So just make sure you are aiming at a surface that you can walk on, and that it is a decent distance away from where you are standing, and then you should be fine.